The present invention relates to thermomechanical methods for improving the fatigue strength and corrosion fatigue resistance of metallic materials (e.g., carbon steel and other steels), particularly components (e.g., sucker rods) made from such materials.
Fatigue strength is traditionally thought of as that stress that a metallic material may support for the time it takes for a fatigue crack to develop at the outer surface of the material and propagate to a depth where the material no longer can support the applied loads. Fatigue strength is usually related to the surface condition of the material. External surface defects such as scratches, nicks, superficial cracks due to heat treatment and other defects result in crack-like stress raisers and act as initiation sites for fatigue. Internal surface defects, including inclusions, behave as stress raisers in the body of the material. Corrosive media also have a strong deleterious effect on the fatigue life of metallic materials through either direct corrosion and/or hydrogen embrittlement mechanisms. This phenomenon is commonly referred to as "corrosion fatigue." Increasing the temperature directly increases the aggressiveness of the corrosive activity.
There are well-known processes which rely on mechanical action to improve the fatigue strength of metallic materials and permit them to be used in parts which are severely stressed during service. One such technique reduces the depth of, or completely eliminates, stress raisers on the outer surface of the material by grinding and/or polishing. This decreases the t/r ratio (depth of discontinuity/radius of curvature of the base) of existing cracks by decreasing the depth of the notches and discontinuities. As a result, the stress concentration factors decrease and fatigue strength of the metallic material is increased.
Studies have shown that crack propagation occurs at the outer surface of the part and propagates normal to the direction of the applied principal stresses. A tensile component in the principle alternating stress system is a necessary condition for fatigue. Thus, the fatigue strength of a part can be improved by creating a region of residual compressive stresses in the surface of the part, thereby eliminating or reducing tensile stresses. Three common methods of inducing compressive stresses in the surface of a material are (1) heat treatment by quenching, (2) chemical additions to the surface and immediate subsurface regions which tend to expand the volume of those regions, and (3) impact on the surface resulting in principal compressive stresses therein. However, none of these methods change the t/r ratio of existing surface discontinuities. They merely prestress the surface of the part in compression so that fatigue will not occur unless the applied alternating tensile stress is sufficiently higher than the existing residual compressive stress built into the part by the process.
The impact treatment of the part's surface will reduce the number of surface discontinuities and introduce beneficial compressive stresses at the surface. Both rolling of shafts and shot peening the surface produce the same general characteristics as impact treatment. However, shot peening does not work on all metal materials and it is quite difficult to control.
Most, if not all, of the conventional methods for reducing fatigue have the following characteristics in common: (1) the effects are restricted to the external surface of the part and to a thin layer immediately below, (2) the volume of material affected is small and the beneficial effects are restricted to discontinuities and notches in the region of the external surface, and (3) the radii of curvature of the notches at the base of the discontinuity remain unchanged.
Conventional treatments of metallic materials to increase fatigue strength also are characterized by the disadvantages that the methods have to be varied for each material depending on its intrinsic characteristics (i.e., melting temperature or transformation point), and that they provide only limited benefit in terms of preventing corrosion fatigue.
An object of the present invention is to provide a process for improving the fatigue characteristics of materials, which is particularly suitable for use on any metallic material and which affects substantially the entire volume of the material subject to fatigue.
Another object is to provide a process for increasing both the mechanical fatigue strength and the corrosion fatigue resistance of metallic materials.